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      <title>CopyOnWriteArrayList和Collections.synchronizedList有什么区别？ - 交互式测验</title>
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            <div class="quiz-header">
              <h1>CopyOnWriteArrayList和Collections.synchronizedList有什么区别？ - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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      <script>
        const quizData = [
  {
    "question": "CopyOnWriteArrayList 的核心工作机制是什么，它是如何实现线程安全的？",
    "options": [
      "通过对所有读写操作使用 `synchronized` 关键字加锁，确保同一时间只有一个线程访问。",
      "通过“写时复制”机制，在修改时创建数组副本进行操作，读操作则访问不变的原始数组。",
      "通过分段锁技术，将数据分成多个段，只锁定需要修改的段，允许多线程并发写入不同段。",
      "通过乐观锁和CAS（比较并交换）操作来避免线程冲突，允许多线程同时尝试写入。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "CopyOnWriteArrayList 的核心在于“写时复制”(Copy-On-Write)。任何修改操作（如 add, remove）都会创建一个新的底层数组副本，所有修改都在这个新副本上进行。操作完成后，再将内部引用指向这个新数组。这种机制确保了读操作永远不会被阻塞，因为它们访问的是一个不可变的、旧的数组快照，从而实现了读写分离。",
    "source": "文档中「工作机制」部分",
    "hint": "这个类的名字“Copy-On-Write”本身就揭示了它的工作原理。"
  },
  {
    "question": "在一个“读多写少”的高并发应用场景中（例如，读取缓存配置），为什么 CopyOnWriteArrayList 通常比 Collections.synchronizedList 表现出更好的性能？",
    "options": [
      "因为 CopyOnWriteArrayList 的写操作更快，内存开销更小。",
      "因为 CopyOnWriteArrayList 保证了强一致性，数据读取总是最新的。",
      "因为 CopyOnWriteArrayList 的读操作是无锁的，可以完全并发执行；而 Collections.synchronizedList 的读操作也需要获取锁，在高并发时会产生线程竞争。",
      "因为 Collections.synchronizedList 在迭代时容易抛出 ConcurrentModificationException 异常。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "CopyOnWriteArrayList 的主要性能优势体现在高并发的读操作上。由于读操作不加锁，多个线程可以同时读取列表而不会相互阻塞。相反，Collections.synchronizedList 对所有方法（包括读方法）都加了锁，在高并发读取时，线程会因为争抢同一个锁而导致串行化，从而严重影响性能。",
    "source": "文档中「性能」部分",
    "hint": "考虑一下两种机制在处理“读”操作时，是否存在“锁竞争”的问题。"
  },
  {
    "question": "关于数据一致性，CopyOnWriteArrayList 和 Collections.synchronizedList 的主要区别是什么？",
    "options": [
      "两者都提供强一致性，保证任何时候都能读到最新数据。",
      "CopyOnWriteArrayList 提供强一致性，而 Collections.synchronizedList 提供弱一致性。",
      "CopyOnWriteArrayList 提供弱一致性（可能读到旧数据快照），而 Collections.synchronizedList 提供强一致性（保证读到最新数据）。",
      "两者都提供弱一致性，无法保证读取到最新的写入结果。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "这是一个关键区别。由于 Collections.synchronizedList 对所有操作都加锁，它保证了任何读操作都能看到在此之前所有已完成的写操作的结果，这称为“强一致性”。而 CopyOnWriteArrayList 的读操作访问的是一个不变的快照，因此在一个写操作完成后，正在进行的读操作可能仍然看到的是旧数据，这是一种“弱一致性”或“最终一致性”。",
    "source": "文档中「一致性」部分",
    "hint": "想一想，当一个线程在写入时，另一个线程读取数据会不会被阻塞？这与数据是否“最新”有直接关系。"
  },
  {
    "question": "在什么情况下，使用 CopyOnWriteArrayList 可能会导致严重的性能问题或资源消耗？",
    "options": [
      "在列表元素非常少，且只有读取操作的场景下。",
      "在数据量巨大且写操作（增、删、改）非常频繁的场景下。",
      "在并发度很低，只有一个线程进行读写的场景下。",
      "在读写操作比例均衡的场景下。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "CopyOnWriteArrayList 的主要缺点是其写操作的成本。每次写操作都需要复制整个底层数组，这会带来两方面的问题：一是如果列表很大，复制操作会非常耗时，导致写入性能低下；二是会产生大量新的数组对象，给垃圾回收带来巨大压力，造成内存消耗显著增加。因此，它完全不适合写操作频繁或数据量庞大的场景。",
    "source": "文档中「内存消耗」和「优缺点总结」部分",
    "hint": "回顾一下“写时复制”这个操作的代价，尤其是在列表很大的时候。"
  },
  {
    "question": "根据文档，以下关于 Collections.synchronizedList 的描述哪一项是正确的？",
    "options": [
      "它只对写操作加锁，读操作是无锁的，以优化性能。",
      "它的读写性能在高并发下通常优于 CopyOnWriteArrayList。",
      "它通过在每个方法上加锁来实现线程安全，适合读写操作比较均衡的场景。",
      "它在执行写操作时会创建底层列表的副本，内存开销较大。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "Collections.synchronizedList 是一个装饰器，它将一个普通的 List 包装起来，并通过在所有公共方法上使用 `synchronized` 关键字来实现线程安全。这意味着无论是读还是写，线程都必须先获取锁，从而保证了操作的原子性和数据的强一致性。这种机制使其在读写操作频率相当且并发度不极端的情况下表现稳定。",
    "source": "文档中「工作机制」和「适用场景」部分",
    "hint": "它的名字 `synchronizedList` 暗示了其实现线程安全的方式。这种方式对读和写操作是同等对待的。"
  },
  {
    "question": "假设一个线程正在对一个包含100万个元素的 CopyOnWriteArrayList 执行 `add` 操作，与此同时，另外10个线程正在并发地读取该列表。会发生什么情况？",
    "options": [
      "所有10个读线程都会被阻塞，直到 `add` 操作完成。",
      "所有10个读线程会立即抛出 ConcurrentModificationException。",
      "所有10个读线程可以正常、无阻塞地继续读取，但它们读到的是包含100万个元素的旧列表快照。",
      "`add` 操作会失败，因为它无法在读取操作进行时获取到写入许可。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "这是 CopyOnWriteArrayList 设计哲学的体现。`add` 操作会在内部创建一个新的、包含100万零一个元素的数组副本。在此期间，所有并发的读线程继续访问原始的、包含100万个元素的旧数组，它们的操作不会受到任何影响，也不会被阻塞。这就是其高并发读取性能的来源。",
    "source": "文档中「工作机制」和「一致性」部分",
    "hint": "`CopyOnWriteArrayList` 的设计核心是读写分离。读操作和写操作是否会相互干扰？"
  }
];
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        let score = 0;
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            const totalQuestions = quizData.length;
            const answeredCount = userAnswers.filter(a => a !== null).length;
            const wrongCount = answeredCount - correctCount;
            const skippedCount = totalQuestions - answeredCount;
            const accuracy = totalQuestions > 0 ? Math.round((correctCount / totalQuestions) * 100) : 0;
            const errorRate = totalQuestions > 0 ? Math.round((wrongCount / totalQuestions) * 100) : 0;

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                '<h2>答题报告</h2>' +
                '<div class="score-summary">' +
                '<div class="score-card">' +
                '<h3>得分</h3>' +
                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
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                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  